The American Naturalist
45: 346-363 (1911)THE BIOMETRIC PROOF OF
THE PURE LINE THEORY11From
a symposium on "The Study of Pure Lines of Genotypes,"
before the
DR. J. ARTHUR HARRIS
CARNEGIE INSTITUTION OF WASHINGTON

I. INTRODUCTION

ON this platform I find myself in a somewhat embarrassing position. A friend
assured me in advance that this symposium would be somewhat analogous to the
country parson's "praise service," and into this pure devotional
atmosphere I must bring a note of agnosticism.

Agnosticism
is a term selected after careful deliberation. Johannsen's propositions are
important—if true—and any candid naturalist must hesitate before opposing a
new theory which may lead to important advances in biology. Agnosticism is the
condition of mind temporarily enforced by the results of my own experiments. If
one is pledged in advance to the pure line theory many of these observations
can be made to confirm Johannsen's conclusions. If one is unprejudiced and
seeks to fit his theories to his observations, rather than to adjust his facts
to his preconceived conclusions, the results are quite as capable of other
interpretation. Possibly more extensive work may show clear confirmation of his
results. Meanwhile I must withhold final judgment, merely stating that my own
work has greatly shaken my confidence in Johannsen's theory.

Here
I do not care to dwell upon details of my own experiments. It seems more
profitable to try and state the fundamental problems of the pure line theory as
they appear to the biometrician and to indicate the methods of work which seem
to him necessary to the drawing of sound conclusions. [347]

II. THE FUNDAMENTAL
PROPOSITIONS OF THE PURE LINE THEORY

Our
symposium has for its subject the Genotype or Pure Line Theory. Some of the
speakers have enthusiastically urged us to replace the words "pure line
theory" by "pure line facts." If this were done there would be
little need for this program. Pure line facts are as yet a very insignificant
part of biological data. The real occasion for this symposium is the pure line
theory—the rank vines which have grown from the nineteen bean seeds which
Johannsen planted in 1901. Biologists would have been little interested by the
statement that selection within the offspring of a single bean has been
ineffective in changing the weight of the seed. It is the daring generalization
of the conclusions drawn from these limited experiments—the curt
characterization of other researches as of no biological significance or their
reinterpretation (from flounder's fins to intelligence in school children) in
terms of the bean experiments, that forces us to take an interest in these
matters.

Our
first problem is to ascertain what these generalizations—the elements of the
pure line theory as contrasted with the pure line facts—are. Our second task
is to try to ascertain in how far experimental facts support the pure line
theory.

2Davenport,
C. B., Science, n. s., 30: 852, 1909.

Davenport2 has given a particularly good outline of Johannsen 's theory:

The
fundamental principle of Johannsen is that, an ordinary frequency polygon is
usually made up of measurements of a characteristic belonging to a non-homogenous
mass of individuals; that it is really analyzable into several elementary
masses each of which has a "frequency polygon" of its own. In each
elementary polygon the variation is strictly due to non-inheritable somatic
modifications, selection of extremes of which has no genetic significance. But
the selection for breeding of individuals belonging to different elementary
polygons, lying, say, at the extremes of the complex, may quickly lead to an
isolation of these elementary polygons, the constituent individuals of which
reproduce their peculiarities as distinct elementary species. [348]

We
recognize three essential propositions:

Proposition
1.—Most
species or varieties are not homogeneous, but are composed of a large number of
minor forms.

The
series of individuals classified as the same variety or race by the
systematist, regarded as homogeneous material for experiment by the
physiologist, lumped together to form a single "population" by the
statistician, is designated by Johannsen as a phaenotype. This phaenotype may
generally be analyzed by pure line breeding into many constant and indivisible
strains known as genotypes.

3"Elemente,"
p. 123.

Systematists
have long regarded certain groups as polymorphie. Aster, Rubus, Salix and Crataegus at once occur to the
botanist and Unio, Salmo and the staphylinids to the zoologist. But the
genotype theory seems to regard systematic polymorphism as a much wider
phenomenon. Indeed one is sometimes assured that every apparently uniform
cultivated variety is a swarm of constant biotypes. Johannsen emphasizes the
generality of heterogeneity. For instance, he says:3

4On
page 121, he remarks
on this point: "Selbst die shonste 'typische' Verteilung beweist gar
nichts in Bezug auf Einheitlichkeit des derart in Erscheinung tretenden
Typus." Professor Johannsen is apparently unaware that this point has been
fully recognized by "Biometriker" for years.

Proposition
2.—These
genotypes are separated generally by differences which are exceedingly minute.

6Spillman,
W. J., AM. NAT., 44: 760, 1910.

Notwithstanding
the constant flood of new species segregated from the classic Linnean groups,
necessitating frequent supplements to "Index Kewensis" and other
works of its kind, many naturalists could hardly understand the small species
discussed by de Vries in his great work. Indeed, many laboratory men hardly
perceived the usefulness of recognizing species—perfectly constant, we were
assured—so closely related that one taxonomist could not identify the species
of another from his descriptions; species so similar that herbarium material
was worthless, and only culture side by side could distinguish them. Yet after
a lapse of only ten years we find de Vries criticized for not recognizing even
smaller divisions than these! Spillman says: "de Vries overlooks entirely
those closely related pure lines, differing frequently only quantitatively, and
in a single character . . . . They not only do not differ in all their
characters as the OEnothera mutants do, but their norms present a regular
series coming under Quetelet's law."6

As
examples of these minute differences both he and Lang7 quote the
"72 Formen einer Population einer gewissen Heferasse" discussed by
Nilsson-Ehle.8

9Jennings,
H., AMER. NAT., 44: 144-145, 1910.

Jennings
says:9

The
work with genotypes brings out as never before the minuteness of the hereditary
differences that separate the various lines. These differences are the smallest
that can possibly be detected by refined measurements taken in connection with
statistical treatment. Johannsen found his genotypes of beans differing
constantly merely by weights of two or three hundredths of a gram in the
average weight of the seed. Genotypes of Paramecium I found to show constant
hereditary differences of one two-hundredths of a millimeter in length. Hanel
[350] found the genotypes of Hydra to differ in the average number of tentacles
merely by the fraction of a tentacle. That even smaller hereditary differences
are not described is certainly due only to the impossibility of more accurate
measurements; the observed differences go straight down to the limits set by
the probable errors of our measures.

Proposition
3.—These genotypes are rigid hereditary units; by a process of mutation one
may give rise to another, but selection within the genotype is incapable of
effecting a change.

This
theory is everywhere so prominent in the writings of the genotypists that
discussion or explanation is superfluous.

Ill.
THE CARDINAL PROPOSITION OF THE GENOTYPE THEORY

10Jennings
(AMER. NAT., 44: 145, 1910) tells us, "The genotype work lends no support
to the idea that evolution occurs in large steps, for it reveals a continuous
series of the minutest differences between great numbers of existing
races."11Johannsen
("Elemente," p. 356) says in criticism of the Lamarckian theory:
"Die Lamarckismus muss kontinuierlich verschiebbare Typen annehmen; wir
finden aber bei genauer Prufung immer und immer wieder Diskontinuitat."

Of
these three essential propositions of the genotype theory of heredity, the
first two might be accepted by Darwinian or Lamarckian or by a member of almost
any school. If the proposition concerning the exceeding smallness of the
differences be true, the theory might seem to present the greatest difficulty
to the de Vriesian,10 for with smaller and smaller genotypes
there is a constant approach to continuity, but we are assured that continuity
is never realized.11

The
third proposition—that genotypic differences are rigid and unchangeable except
by mutation—is therefore the essential one. The most obvious way in which this
hypothesis can be tested against concrete facts is to determine the effect of
selection upon genotypes.

The
very heart of the pure line theory is the proposition [351] that selection
within the pure line is ineffective.12

The
strenuousness with which this has been maintained has even engendered in some
minds the opinion that selection has no rôle at all to play in evolution or in
practical breeding. The attitude of many appears to be that Darwin was quite
mistaken when he wrote, "The key is man's power of accumulative selection:
nature gives successive variations; man adds them up in certain directions
useful to him."

Darwin
said, "If selection consisted merely in separating some very distinct
variety, and breeding from it, the principle would be so obvious as hardly to be
worth notice." Fifty years after this was written we hold a symposium to
celebrate the discovery that selection is after all merely the isolation of
distinct varieties!

Was
Darwin right or wrong? Have all practical breeders except those at the oft-quoted
Svalöf station been chiefly occupied in wasting their time for the last fifty
years? These are very important questions.

13To
be acceptable, the evidence must be quantitative; the observations must either
be numerous enough that variations due to uncontrollable factors will average
out, or the experiments be conducted with such refined technique that
environmental influences are entirely excluded; the statistical reasoning
concerning the observations must be logically sound.14"A
biotype is a group of individuals which do not differ from one another in any
hereditary quality and which therefore constitute a pure race."—Shull, C.
H., Am. Breed. Mag., 1: 100, 1910.15"In
a given 'pure line' (progeny of a single individual) all detectable variations
are due to growth and environmental action, and are not inherited."—Jennings, Proc. Am. Phil. Soc., 47: 521, 1908.
"The
standard deviation and coefficient of variation express in a pure race mere
temporary conditions of no consequence in heredity. If we could make all
conditions of growth and environment the same throughout our pure race, all the
evidence indicates that the standard deviation and coefficient of variation
would be zero, and this is the positive value of their assistance in
determining what shall be the characteristics of the progeny."—Jennings,
AMER. NAT., 43: 333, 1909.
"Wenn
es gelänge, für aIle Individuen einer reinen Linie absolut gleiche Lebenslage
zu schaffen, müsste die
Standardahweichung gleich sein."—Mimer, T., Arch. Rassen- u. Gesells.-Biologie, 7: 437-438,
1910.16For
instance, he ("Elemente," p. 162) refers to the fact that Hallet was
unable to improve Le Couteur's wheat, although he had succeeded in improving
seventy other samples from all halts of the world, and explains it by the
assumption that in every case the seventy series of wheat were mixtures of
biotypes while Le Couteur's was a
pure line. This may be true, but what is it worth as scientific evidence?
17In
working over the literature of the pure line theory the lover of fair play is
sometimes on the verge of losing his patience, for although the experimental
data—at least those which are confided to his reader—upon which Johannsen
grounds his own theory are very slender, he is unsparing in his criticism of the
pioneer studies which have made his own work possible. Such bald statements as
("Elemente," p. 285), "Alle solche Schlüsse sind aber für die
eigentliche Erblichkeitsforschung ganzlich ohne Wert," seem to have little
of profit to contribute to science. Johannsen's ipse dixit has been taken as
gospel. Woltereck (Verh. Deutch. Zool. Ges., 1909: 115) says,
"Dieses Resultat erschuttert ernstlich die grundlagen der statistischen
Variations; und Erblichkeitforschung, wie sie von die Galton-Pearsonschen
Schule betrieben wird." A. Lang (Verh. Deutch. Zool. Ges., 1909: 24) asserts,
"Die biometrischen Forschung arbeitet mit unreinen material." Römer (Archiv
f. Rossen- u. Ges.-Biol., 7: 427, 1910) tells us, "Variabilitatstudien sind bis
in die neueste Zeit meist an Material ausgefuhrt worden, dessen Einheitlichkeit
jeweils als sicher angenommen wurde, das aber nach dew jetzigen Stande der
Wissenschaft als unrein angesehen werden muss. Dies tritt besonders hervor bei
den veilen Untersuchungen der Biometriker."18This
is one of the facts which has led the biometrician to discuss probabilities
while biologists in general clamor for certainty in the individual instance.
One of the results of recent experimental work that has been hailed with the
greatest enthusiasm is that two individuals may be identical in external
appearance and yet produce entirely different offspring: in short, that some
(somatic) variations are and some are not inherited. The experimental data
collected on this point both by pure line and by Mendelian researches are of
high value, but those who hail them as novel simply parade their ignorance of
much of the pioneer work in variation and inheritance.19Pearl,
B., and F. M. Surface, "Inheritance of Fecundity," Bull. Me. Ag. Exp. Sta., 166, 1909. Pearl, B.,
and F. M. Surface, "Is there a Cumulative Effect of Selection?" Zeitschr.
Ind. A btamm.- u. Verebungsl., 2: 257-275, 1909.20Surely
we can all agree that the population is to be an apparently homogeneous one, i.
e., such that all the individuals would be classified together by a keen
taxonomist, if this is not the case, if by definition, "population"
means to the pure linist a mixture of several conspicuously different things,
there seems little need for further discussion.21Of
the seed he says, "Der Ausgangspunkt dieser Unteisuehtmgen war eine
gekaufte Partie, etwa 8kg, brauner 'Prinsessbohnen,' wohl eine der altesten
Kiuppbohneu unten den vielen Kulturformen von Phaseolus vulgaris. Die
betreffende Ware war ausgezeichnet schön und so gleichmässig, wie es überhaupt
hier erwartet werden konnte."

The
burden of proof obviously lies on the genotypists.13 Much of the
evidence offered is most general and not at all unzweideutig. Indeed, when closely
analyzed much of the reasoning reduces to a circle of three arcs each of one
hundred and twenty degrees:

1. Definition.—A genotype
or biotype is an organic unit, reproducing itself constantly14 except for the transitory, non-inheritable modifications due to environmental
influence.15 It is not capable of change by selection. [352]

2. Observation.—Selection
has never been known to produce a change in a genotype. Whenever, as is often
the case, selection does result in modification of type this proves that the
material considered was impure—that more than one genotype was originally
present—or that others arose by mutation, and entirely irrespective of
selection.

3. Conclusion.—It
is therefore
proved that selection can not modify the characters of a genotype.

Johannsen
has written a very thick and a very convincing-looking book, but if one pins
himself down to the task of going from cover to cover he finds that an
unfortunate amount of the evidence reduces to this kind of reasoning—in short,
to no critical evidence at all.16 But behind this citing of examples
which are not inconsistent with his theory although they prove nothing
concerning it; besides this reiteration of testimony which merely excites in
the minds of the court-room spectators suspicions concerning the integrity of
the defendant without entitling the plaintiff to a verdict before an impartial
jury,17 there are certain direct experimental studies [534] which have been adduced in support
of the genotype theory. These arguments and the evidence upon which they rest
must be examined. For convenience of treatment I do this under three
propositions concerning selection, which seem so reasonable that I believe few
biologists will feel inclined to deny their soundness. They are at least so
reasonable that no worker can afford to leave them out of consideration.

A.
Characters which are not Inherited at all can not be Taken to Prove that
Selection in General is Ineffective

This
is a point of great importance, generally ignored by pure-linists.
Biometricians have long known that of the variations of any character whatever
not all are inherited.18 They have also learned that variations in certain
characters are not inherited.

Suppose
now that one takes a character which gives no correlation between its degree of
development in [354] parent
and offspring in a population and selects to increase or decrease it. He will
get no result of selection. If now he takes the same character and selects from
the plus and minus variations within a pure line, he will again effect no
change by selection. Does either of these cases prove that selection in general
is ineffective? Or does the second support in any way Johannsen's genotype
theory of heredity? Certainly not.

Certain
important work of Pearl and Surface seems to me to deserve mention in this
connection.19 These researches are sometimes referred to as
furnishing evidence against the possibility of improvement by selection, and
this they do so far as the character with which they have dealt is concerned.
In the generalization of their results, however, the greatest caution must be
used.

From
two series of experiments with the same strain of Barred Plymouth Rock fowls
they show that there is little hope of increasing the egg-laying capacity by
direct selection for fecundity. These results are doubtless of much practical
importance. Biologically they are of interest in confirming the results of
other biometric studies which have shown that for man, horse, swine and mice
fertility is very slightly inherited in the population. To consider them as
indicating that selection in general is ineffective would be a very grave
error, for fertility—so far as we may judge from the statistics so far
published—seems to be a character sui generis in respect to
inheritance. To cite these results in support of Johannsen's genotype theory of
heredity, as has sometimes been done, is absurd.

Is
it not possible that Johannsen's results with beans may be due to seed weight
being a character which is not inheritable at all in the population, and which
can not, therefore, reasonably be expected to be inherited within the pure
line? [355]

Biologists
will agree, I believe, that to test critically the effectiveness of selection
in the population and in the pure line, the experimental material must be an
apparently homogeneous wild species or a garden variety the individuals of
which are not differentiated into subraces by characters other than those under
consideration.20 Conclusions drawn from any experiments in which
these simple precautions are neglected seem of doubtful value.

From
Professor Johannsen's first memoir, that of 1903, we have no reason to suspect
that his material is not, so far as the biologist can judge, homogeneous.21 We are told nothing of any vegetative differences seen during the two
generations grown in 1901 and 1902. Apparently all the numerous reviewers have
considered his material perfectly homogeneous except for differentiation into
genotypes with respect to seed characters.

In
his book, however, one notes with some surprise the casual information
("Elemente," p. 311) that his Pure Line I also has curiously bent
seeds, a special "Verhalten" in germination and a "groben
Habilus" in the vegetative organs. Indeed Johannsen states that from the
form and method of germination, etc., of a seed—even though a strong
"minus Abweicher"—he can generally recognize an individual belonging
to Line I.

These points should have been made clear
at the beginning. If Professor Johannsen's lines really differ in their
vegetative characters, so, for instance, that they can be distinguished as they
grow in the field, it seems to me that their significance for the efficiency of
selection is [356] greatly reduced. We do not know to what extent the
differences in seed weight which give the low correlation in his population are
due to the mixture of races slightly differentiated with respect to their
vegetative characters. If this differentiation be considerable, the seed weight
character with which Professor Johannsen has chiefly worked, may not be
inherited at all in the population providing this population be one composed of
individuals with the same vegetative characters. It is not sufficient to be
assured that these classic beans differ "nur (oder fast nur)" in seed
characters; more detailed information is much needed, and until it is
forthcoming I must differ from most biologists in my opinion as to the
importance to be attached to the conclusions drawn from them.

B.
Improvement for any Single Character can not be supposed to be Unlimited

22The
principle, however, has been clearly seen by some biologists. For instance, in
his "Foundations of Zoology," Brooks says (p. 165): "A breeder
of domesticated animals or of cultivated plants, who devotes his attention to
one or two characteristics, must soon reach a point where no further
improvement is practicable unless the species is at the same time greatly
modified in many other respects." And again (pp. 177-178), "No one
can dispute the well-known fact that this sort of pedigree selection for a
single point quickly grows less and less effective, and soon reaches a maximum;
but this is no proof of any 'principle of organic stability',' or anything else
except the truth that long ages of natural selection have made the organism
such a unit or coordinated whole that no great and continuous change in one
feature is possible unless it be accompanied by general or constitutional
change."23East,
E. M., "The Role of Selection in Plant Breeding," Pop. Sei. Mo., 77 :
198-199, 1910.24That
changes due to selection are at first rapid and then slower has long been
recognized. Indeed, as early as 1869 Hallett stated as two of his laws of the
action of Selection, "The improvement which is at first rapid, gradually,
after a long series of years, is diminished in amount, and eventually so far
arrested that, practically speaking, a limit to the improvement in the desired
quality is reached. By still continuing to select the improvement is maintained
and practically a fixed type is the result."
Darwin
's views on this question are partly expressed in a letter of 1869 to Sir
Joseph Hooker ("More Letters," 1: 314), "1 am not at. all
surprised that Ilallett has found some varieties of wheat could not be improved
in certain desirable qualities as quickly as at first. All experience shows
this with animals."25Pearl,
B.., and P. M. Surface, "Experiments in Breeding Sweet Corn," Me. Ag.
Exp. Sta. Bull., 1910.26Judging
from the account of the difficulties of growing sweet corn which the authors
give us.

This
is a fundamental consideration too often neglected.22 A wheat is selected
up to its maximum productiveness, perhaps by getting the uppermost attainable
limit at one choice from a large field. Then because it can not be made to
yield all grain and no stubble we are told that selection can only isolate
already existing types. A sugar beet can not be all sugar and the cow can not
give pure cream.

In
arguing for Johannsen's theory East23 concludes that since Illinois
is no longer making progress in high [357] and
low oil and protein selection in maize, their work has been merely the
isolation of pure and constant strains—"sub-races "—with the
characteristics in question as strongly developed in the beginning as we now
find them, but continually intererossing. The case is too complicated for
discussion in detail, but certainly the fact that the characters can no longer
be increased by selection24 is no strong argument for the biotype
idea. Under its present morphological and physiological organization we have no
reason to suppose that the corn grain can be made to contain as much oil as the
castor bean.

Again
Pearl and Surface25 announce concerning their selection work with
corn,

We
find the results of this experiment or investigation to be very difficult (if
not altogether incapable) of rational explanation in accordance with the
biological implications of the "law of ancestral inheritance" and
conclude that the results agree better with the genotype theory of Johannsen
than with that of the cumulative theory of selection with, of course, the
limitations implied by the fact that it is an open fertilized plant.

What
Pearl and Surface have actually done is to take a desirable sweet corn which
they for convenience designate as Type I, and attempt—with initial success—to
improve it for yield in ears and stover, for configuration of ears, and
especially for earliness. But this Type I corn is descended from a few ears,
the offspring of which have been grown in Maine for fifteen to twenty-five
years. The variety originally introduced must have been an [358] early
one as compared with sweet corn in general, to be able to survive at all in
Maine. During the fifteen to twenty-five years the ancestors of the Type I corn
were grown in Maine it must have been26 subjected to an occasional
natural selection, for seed could be taken by the farmers from only plants
which had ripened their ears. The somatic organization of some plants is such
that they require only a few hours for their life cycle, but so long as sweet
corn has the general characteristics of root, shoot and leaf that identify it
as Zea Mays it seems reasonable to suppose that there is some limit to the reduction of the
time required for germination, growth and fruiting—an irreducible minimum
beyond which selection can not carry it. Surely the fact that Pearl and Surface
could not continually reduce the time required for growth while at the same
time maintaining a selection for yield of ears and stover may indicate that the
irreducible minimum for earliness has been reached in a variety of the physical
type they wish to breed. Speaking for myself alone, I must say that the data
before us prove nothing against the theory of cumulative effect of selection,
and they certainly do not furnish any critical evidence for the Johannsenian
theory.

27Pearl
and Surface, Bull. Me. Ag. Exp. Sta., 166 55.

It
seems to me that Pearl and Surface again tacitly make this unjustifiable
assumption that the modification attainable for any single character is
practically unlimited when they consider that their failure to increase egg
production by selection is a legitimate argument against the potency of
selection. Indeed they say of "200 egg hens," which lay an egg fifty-five
per cent. of the days of the year, "This figure is of some interest as
indicating what a relatively small proportion of the theoretically maximum
character is being selected to, when 200-egg birds are bred."27

But why, pray, is two hundred and sixty-five
and a quarter eggs per year the theoretical maximum? One [534]
ignorant of the physiology of reproduction in the domestic fowl might
innocently suppose that even a hen needs a rest. If this be true, may it not be
that 200 eggs is about the attainable maximum (the physical or physiological
limit of the organism) of this variety under the environmental conditions
available and that the Maine strain of poultry will not do better than it has?
If this is not the attainable limit, why not assume over an egg a day as the
theoretical maximum?

C. Selection can not
in general carry a Character beyond a Degree Consistent with the Optimum for
Maintenance and Reproduction

This
proposition is perhaps in a sense explanatory of the one immediately preceding.
A characteristic is not independent of, but correlated with the other
characteristics of the organism, and if it increases or decreases unduly they
must also change or the organism be made more or less unfit for survival.

Have
those who claim to have found selection ineffective been selecting against the
morphological or physiological balance of the organism, that is in a manner to
render the organism less capable of maintenance, growth and reproduction?

If
this be true their failure to obtain results will be in some measure explained.

28The
criticism presented here must not be interpreted as drawing into question the
scientific value of the data or the practical importance of the results of the
studies criticized, or be extended to other work of the same authors, but
is to be limited to the question of interpretation in relation to the pure line
problem.

A
possible illustration of this case may be furnished by the work of Pearl and
Surface on egg production in the domestic fowl. Their work is again chosen not
because of any malicious desire to differ from them28 in
interpretation, but because in a brief discussion of the evidence for the
genotype theory one must confine his attention to the most important of
Johannsen's supporters.

The
data are: (a) The results of an eight years' selection [360] for high egg
production; (b) a correlation between the egg production of thirty-one
individual mothers and the egg production of their daughters, and the
comparison of the egg production of these daughters with that of a large number
of pullets of unregistered female parents.

We
note the following details:

29"The practise in breeding was to use as mothers of the stock bred in
any year only hens which laid between November 1 of the year in which they were
hatched and November 1 of the following year, 160 or more eggs. After the first
year, all male birds used in the breeding were the sons of mothers whose
production in their first laying year was 200 eggs or more. Since the normal
average annual egg production of these birds may be taken to be about 125 eggs,
it will be seen that the selection practised was fairly stringent." Zeit.
Ind. Abst.- u. Verebungsl., 2: 261, 1909.30From
a knowledge of the biometric work of the last several years this is just the
result which one would have expected to get.

1.
During the eight-year selection experiment29 some unfavorable environmental
accidents occurred in certain of the laying years. The averages for these years
are perhaps too low, and both the actual means and a series of corrected means
are given. The corrected means show an insignificant increase, but the
unmodified means show a pronounced decrease in mean number of eggs as the
result of the eight year selection.

2.
In correlating between the egg production of the 31 highly selected mothers and
their 217 daughters there is not trustworthy evidence of any relationship
between the fertility of the mothers and that of their daughters.30If
these constants show any deviation from 0 whatever it is on the negative side.

3.
In comparing the daughters of these "200-egg" hens with three other
series of the same strain but not of such highly selected female parentage,
both for winter and spring egg production, it is shown that in five cases out
of six the offspring of less highly selected parentage are better layers than
those of the less stringently selected parents.

Thus
all three comparisons indicate that the high laying mothers tend to produce low
laying daughters; selection to increase egg production actually decreases it. [361]

31The argument that this observed decrease as the result of selection to increase egg production is due to chance must rest chiefly on one or both of two assumptions. First, that the eight-year selection experiment is absolutely untrustworthy because of the accidents which may have affected the egg production in certain years adversely. Second, that 31 mothers is a number entirely too small to give significant results in the case of a character like fecundity. These admissions would vitiate entirely any conclusion concerning selection to be drawn from these experiments.32To me it seems that some of Pearl and Surface's published data are most suggestive of the nature of this factor, but they doubtless have in progress experiments that will throw light on these matters and biologists will await their results with interest.

Such
a run of results as this can hardly be due to chance.31 They
indicate rather the presence of some as yet undetermined physiological factor.32

Candidly
viewed and considered in comparison with other biometric work on the
inheritance of fertility and fecundity, I think these experiments can not be
held to be strongly opposed to the theory of the effectiveness of selection in
general. However this may be, they certainly afford no substantiation for
Johannsen's genotype theory of heredity.

IV. SUMMARY AND
CONCLUSIONS

By
the genotype theory of Johannsen one understands the following propositions:

An
apparently uniform population or phaenotype is generally not homogeneous, but
is composed of a large number of differentiated types, which are
designated—within limitations to be laid down immediately—as genotypes.

Externally
the genotype can not be distinguished from the phaenotype. Both may have normal
variation curves, but while that of the phaenotype may by proper selection be
broken up into constituent genotypes, the variation curve of the genotype can
not be modified by selection. In short, the genotype is from the standpoint of
heredity a rigid unit. All individuals belonging to the same genotype have the
same potencies as parents. Only discontinuous segregations or
transformations—mutations—may modify them. [362]

The keystone of the pure line arch is the
proposition that selection is ineffective except as a means of separating
already existing genotypes. If this keystone-proposition be not sound the whole
structure of the theory crumbles.

The propositions of the genotype theory are such
that scientific proof or disproof is rendered particularly difficult. By theory
selection can not effect a change in a pure line; by a slippery process of
reasoning in a circle any results attained by selection are at once discredited
by the assertion that the original material was impure. If, on the contrary,
any selection experiment is ineffectual it is by some process of reasoning
quite incomprehensible to some of us, at once chalked up to the credit of the
new theory. If heritable differences appear within a pure line known to be so,
these results are also discredited by the assertion that the observed change is
a mutation or has been produced by the action of the environment. Truly the
unbiased investigator is between the devil and the deep sea!

33In
offering this criticism I wish to express the highest admiration for Professor
Jennings's two memoirs on variation, heredity and evolution in the protozoa.
The coupling of refined statistical with careful experimental methods in the
investigation of these organisms marks a great advance in biology.34Naturally,
this is purely a matter of interpretation, and does not diminish in the
slightest degree the value of the work.

The
actual experimental data upon which the genotype theory rests are as yet few.
Johannsen's conclusions for beans depend chiefly upon the offspring of only
nineteen seeds, and so far as I am aware no other investigator has confirmed
his results on Phaseolus. Hanel had only twenty-six original Hydra, and Pearson's analysis
of his data with more adequate methods than he used, evidences against rather
than for the genotype theory. Jennings gives us the records of only six
selection experiments involving altogether only a few actually selected Paramecia. Considering the large
environmental and growth factors, his conclusions can not be considered as
beyond question.33 The work of Pearl and Surface with poultry and
maize seems to me to have [363] no critical bearing on the pure line problem.34 This is also true of numbers of other smaller experiments which can not be
cited.

If
one turns from the strictly pure line side of the problem to the more general
questions of the "something" or "Etwas" in the germ plasm
which determines in large degree the somatic characters of the individual
which develops from it, one can only suggest that nothing whatever is explained
by giving another name to a well-known fact. Ever since the time of Darwin, and
before, we have known that there was "something" in the germ cells
which determined the character of the offspring. We have had a dozen different
names for this something, and by adding a thirteenth, "Gene,"
Johannsen has merely burdened us with another cloak for our ignorance.
Unfortunately biological closets are full of such cloaks, once in fashion—now
out.

Finally,
I must make my own position quite clear. With Professor Jennings's contention
that pure line cultures are of fundamental importance in many fields of
physiology and genetics, I am in hearty agreement. Like other breeds of facts,
"pure line facts" can not become too abundant. Indeed, a priori, I am
not opposed to the genotype theory. As a theory it is most attractive, but one
can not accept it without proof on that account. Personally, I am one of
"that last small remnant" who believe that in a problem of this kind
the proof must be biometric. This means merely three things. In so far as the
nature of the material permits, all the data considered must be quantitative.
The data must be numerous enough that biological relationships will not be
obscured by the errors of random sampling. The data must be analyzed by
logically sound methods.

Judged
by these standards, I must express the conviction that as yet there is no
adequate justification for the genotype or pure line theory.